for adding black bar so picture will not be distorted.In this case if subtiltes are rendered and are placed in this added black bar, when :1: is not used, OSD text(subs) are not cleared so they hang until they are overwritten with another ones so garbish is displayed (this zone is not refreshed).When "-ass" parameter is used, :1: param is not working as expected so text hangs on the screen.I read lot of complainms that "-vf softskip,expand=::::1:16/9:4" doesn't work for some so now I know why. It works only when -ass subs are disabled